I've changed the width of my sidebar to 400px because I think that is better for some of my widgets. Adding a second column would take away too much of the space left for the actual forum, so that would not be optimal in my case. I could see that it would be great in some cases, especially with high screen resolution. In such cases one could have one left and one right sidebar.
I agree that changing the sidebar from category to category would reduce the consistency and hence perhaps make the forum less user friendly, but that depends on what changes are made and how it fits with the overall forum. In some cases being able to change the sidebar could make the forum more user friendly because the sidebar content would be relevant all the time. In my case it could just be a matter of changing the pictures in the slideshow widgets (from music related to car related depending upon which category one is in), or changing the type of news (different news search keywords) that are displayed in a RSS widget, and similar changes, which really does not change the consistency of the sidebar since the same widgets are there, just fine-tune them to be more relevant.
Some admins might abuse the "freedom" and confuse the members by altering the sidebar too much from category to category, but they are probably already making lots of lay-out mistakes already. I guess it's a matter of finding the balance between reducing the probability that admins will make lousy forums and giving admins the possibility of making really good forums.
Have a great weekend!